The present invention relates to methods for heating a film on a substrate. For example, in the manufacture of a flat panel display, a film of amorphous silicon is placed over a glass substrate. The silicon is subjected to normal semiconductor wafer processing techniques to form transistors along the top or bottom and one side, with an array of lines extending through the display from the transistor to create a grid of interconnections. Each element in the display is activated by turning on the appropriate two transistors whose lines intersect at that element. When amorphous silicon is used, the size of the display is limited because an amorphous silicon transistor has a limited distance of a line that it can drive.
Crystalline or large grain polycrystalline silicon is preferable to amorphous silicon because it has a mobility on the order of 300-400, compared to a mobility of approximately one for an amorphous silicon transistor. Amorphous silicon can easily be applied in a film on a substrate of glass. To convert the amorphous silicon into polycrystalline silicon, it must be subjected to high temperatures (in excess of 600.degree. C). Unfortunately, glass will warp at such a high temperature. Quartz can be used as a substrate since it will not warp at this high temperature, but it is much more expensive than glass. Thus, flat panel displays are either small and use amorphous silicon on glass, or are larger with crystalline silicon on quartz, or use some other techniques. Other techniques include liquid crystal displays (LCD), which do not use active transistors for switching and consequently have a slower response.
One method used to recrystallize amorphous silicon without substantially heating the glass is the use of a laser beam. Unfortunately, because the laser beam is small, it must be raster-scanned in an overlapping pattern, which may result in non-uniform heating and takes a lot of time and power.